The present invention relates to tires, and more specifically, to an improved design of tire bead intended to make the tire easier to remove from a rim.
The role of tire beads is to "secure" the tire on the wheel rim on which it is mounted. Toward this end, the carcass cords all meet at the bottom part of the bead where they are firmly anchored so that the carcass can withstand tensile loadings in service.
In addition to the stresses inherent with the actual use of the tire, the latter needs also to be capable of withstanding an unspecified number of removals followed by refittings for its continuing use.
Removal of a tire from its rim involves applying quite a high force to the bead near the flange of the wheel rim or just beyond it. This force is oriented parallel to the axis of rotation and is usually applied by a press to successive portions of the circumference of the bead that correspond to circular arcs of approximately 60 degrees. This is the first phase of removal, the purpose of which is to lift the bead, that is to say, to move it off its seat or radially inward wall by moving it away from the flange of the wheel rim and bring it as far as the central well of the wheel rim. During this first phase, the tire bead is subjected to relatively localized and very high extensile forces.
Next, levers are generally used to force the bead over the wheel rim flange. Effectively, in the case of one-piece wheel rims (this case being by far the most common for passenger car tires), the shape of the wheel rim is designed to allow fitting and removal of the tire by ovalizing the bead without increasing its perimeter. This particularly governs the design of the central mounting well and of the flanges which laterally border the wheel rim and define the mounted position of the bead. During this second phase, the bead is subjected to an overall deformation which is far less penalizing than the stresses that occurred during the first phase.
It is well known that if a tire is removed carelessly there may be irreversible damage to the bead (breakage of carcass cords, tearing of the heel of the bead which is usually retained by a hump, etc.) as a result of the localized and high extensile forces imposed during the first phase of the removal.
This is what is likely to happen if attempts are made to lift the bead all at once with the press by bringing about a high localized offset of the bead relative to the flange of the wheel rim. It is common for the beads to stick tightly to the seats and flanges of the wheel rim after a few months or years of use. This substantial and localized offsetting causes a lengthening of the bead wire, and this may lead to irreparable damage to the bead of the tire.